1. Technical Field
This invention relates generally to hypodermic injection devices, and more particularly to needleless injection devices that avoid piercing the skin by utilizing a high pressure jet to pass the medication or other substance through the skin.
2. Background Information
Needleless injection devices reduce the relative risk and discomfort of puncturing the epidermis with a hypodermic needle. In addition, they are more easily used by persons of limited skill. Thus, their use has become of increasing interest, particularly by such users as those having frequent insulin injections.
One existing needleless injection device is described in U.S. Pat. No. 4,680,027 to Parsons et al. It employs a disposable syringe that is filled by the user with a selected amount of medication. Then a separate power supply mechanism is attached to the instrument and triggered to drive a plunger against the medication. This produces a high pressure jet for injection purposes.
Although effective in many respects, the separate power supply mechanism reduces the portability and convenience of the Parsons device. Consequently, it is desirable to have a needleless injection system that overcomes this concern.
U.S. Pat. No. 4,596,556 describes a hypodermic injection apparatus having a spring-loaded firing mechanism that operates to release CO.sub.2 from a capsule. This propels the capsule and a plunger toward a discharge aperture to discharge medication through the aperture. However, in being self-cocking, reloadable, and reuseable the device is somewhat complicated.
U.S. Pat. No. 3,688,765 describes another device that is intended for portable use. However, it uses a syringe in the form of a prefilled, sealed ampoule that precludes dosage selection at the time of use. In addition, the detachable syringe, disposable CO.sub.2 unit, and re-usable pressurizing unit represent three separate components with which the user must contend. Consequently, it is desirable to have an injection system that overcomes these concerns also.
In this regard, it is desirable to have a pre-sterilized, disposable device for single injection use that has its own power source while being small and light enough for the user to carry along during the day. The device should be such that the user can fill it with a selected dosage conveniently and accurately from existing medication vials, and it should be conveniently and safely operated without the need to cock a spring as the source of power.